If your scenario includes zooming and scrolling, the Canvas solution will no-longer be easy to implement.
That's why I offer you another solution - represent the line annotations with LineSeries (or ScatterLineSeries, depending on the chart axes). This gives you the advantage of out-of-the-box zoom and scroll support.
To determine the values of the data points of the line series, you can use the RadCartesianChart.ConvertPointToData. For more information you check this
I have attached a sample project. The user can create a line, by clicking twice on the chart.
Let me know if you need further assistance in implementing this in your project.
the Telerik team